The present invention relates to integrated circuit device testing and in particular to controlling temperature in a device under test (DUT) during testing.
Typically, in integrated circuit (IC) processing, semiconductor wafers and packaged ICs are subjected to a series of test and evaluation steps. For each step, the wafer or packaged circuit device under test (DUT) is held in a stationary position at a process station where the process is performed. In wafer testing, the wafer is typically held stationary relative to a vacuum support surface of a prober machine which electrically tests the circuits on the wafer. The prober includes a group of electrical probes in a probe head which is positioned such that the probes are brought into contact with predetermined contact points on individual circuits formed in the wafer. The probes, in conjunction with a tester, apply predetermined electrical excitations to various predetermined portions of the circuits on the wafer and sense the circuits"" responses to the excitations. In packaged IC testing, the IC is typically held in a test socket mounted on a test board. The electrical stimulus signals are applied to the DUT and response signals are received via the socket and the test board.
The test system used to test a wafer or a packaged device can also include a temperature control system which maintains the temperature of the wafer or DUT at a desired set point temperature. Temperature control systems for a wafer can take several forms. For example, the chuck can be outfitted with electrical heaters which are controlled to provide heat to the chuck and raise the temperature of the wafer. The chuck can also include a heat sink for removing heat from the chuck. The temperature control system can also include a means for circulating a fluid through the chuck to cool and/or heat the chuck.
In wafer testing the set point temperature of the chuck is selected based on a desired testing temperature for the wafer. The chuck typically includes a temperature sensor mounted within the chuck in close proximity to its top surface and, therefore, the wafer mounted thereon. Since the temperature sensor is located close to the wafer, it is assumed that the temperature at the top surface of the chuck is the same as the temperature of the wafer circuit being tested and that any difference in temperature between the two is negligible.
In many modern circuit testing settings, this assumption can lead to inaccurate testing results. For example, in many circuits, power dissipation is relatively high. This results in substantial localized heating of the circuit or adjacent circuits when the circuit testing excitation signal is applied by a tester operating with the prober. The result is that the circuit is actually tested at a much higher temperature than the temperature set by the system, and the accuracy of the test is reduced.
In a system for testing packaged ICs, the temperature control system can provide a temperature-controlled stream of fluid or gas, e.g., air, directed onto the DUT package. An example of such a system is the ThermoStream system sold by Temptronic Corporation of Newton, Mass., the assignee of the present application. The temperature control used in the ThermoStream system is described in U.S. Pat. No. 4,734,872, assigned to Temptronic Corporation, the assignee of the present application, and incorporated herein in its entirety by reference. In the ThermoStream system, the set point temperature of the DUT is maintained by simultaneously monitoring the temperature of the air stream and the temperature of the DUT package. In many applications, even with the temperature probe on the package, some level of inaccuracy, albeit small, can be introduced.
It would be beneficial to have a temperature control system for controlling the temperature of a wafer or device under test in which temperature sensing is realized at the actual circuit being tested.
The present invention is directed to an apparatus and method for controlling the temperature of a device under test (DUT) having an integrated circuit chip die which overcomes the drawbacks of the prior art. The apparatus of the invention includes a temperature sensing device integrally formed in the integrated circuit chip die. A sensing circuit receives from the temperature sensing device a signal indicative of temperature of the integrated circuit chip die. A temperature control medium is in thermal communication with the DUT. The temperature of the temperature control medium is controlled by a temperature control system. The temperature control system uses the signal indicative of the temperature of the integrated circuit chip die to control the temperature of the temperature control medium such that the temperature of the integrated circuit chip die is controlled.
In one embodiment, the temperature control medium is a stream of a temperature-controlled gas such as air which is directed onto the DUT. The temperature control system controls the temperature of the air to control the temperature of the DUT.
The temperature sensing device formed in the integrated circuit chip die can be a temperature-sensitive diode. The diode is formed in the chip along with the other circuitry. An excitation current can be driven through the diode by the DUT testing system, and the resulting voltage drop across the diode can be sensed. In such a diode, the voltage drop across the diode changes with temperature. Therefore, the sensed voltage drop can be used to generate the signal indicative of temperature of the integrated circuit chip die.
The DUT can be a packaged integrated circuit with an internal chip die. The device can include conductive pins which provide the interface between the internal circuitry and external devices and systems.
In one embodiment, the sensing circuit can be part of a testing circuit in a system used to test the DUT. To test the device, the testing circuit applies test stimulus signals to the DUT, and receives test response signals from the DUT, via the package pins. The testing circuit can be coupled to the temperature control system. The signal indicative of temperature of the integrated circuit chip die, received via one or more package pins, is used to generate a control signal which is applied to the temperature control system and is used by the temperature control system to control the temperature of the temperature control medium, e.g., the air stream.
In one embodiment, the temperature control process used in the invention directly controls the temperature of the temperature control medium, i.e., air stream, to control the temperature of the DUT. That is, in the air stream system, the temperature of the air stream is sensed, and the sensed temperature is used to control the temperature of the air stream to maintain it at a desired set point. If the air stream temperature is at its set point and the diode output signal does not indicate that the DUT is at its desired set point temperature, then a new air stream set point is selected. This continues until the desired temperature is reached. Hence, the set point for the temperature control medium, i.e., air stream, is in general different than the set point for the DUT.
This type of control can be regarded as a xe2x80x9cdual-loopxe2x80x9d control in which two types of feedback are used to achieve the ultimate desired output. Such dual-loop temperature control, which can be used in accordance with one embodiment of the invention, is described in, for example, U.S. Pat. No. 4,734,872, entitled, xe2x80x9cTemperature Control for Device Under Test,xe2x80x9d issued Mar. 29, 1988, assigned to Temptronic Corporation, and incorporated herein in its entirety by reference.
The temperature control approach used in accordance with the invention is used to precisely control the temperature of the device being tested, with temperature sensing being realized at the circuit under test, not at a relatively remote location. As a result, the temperature control tests performed in connection with the temperature control of the invention are more accurate than tests performed using prior approaches to temperature control.